Noise-cancelling headphones have been commercially available for many years, to provide personal listening environments in which ambient acoustical noise is reduced at the listener's ears by cancelling, as opposed to insulating the listener from, the noise.
Like ordinary headphones, noise-cancelling headphones are adapted to be plugged into devices that output, into the headphones, desired electrical signals representative of sounds such as music or speech that are of interest to the listener, the desired electrical signals having been amplified sufficiently in the device to drive the speakers in the headphones. For example, headphones can be connected to smartphones, for listening to music that is being streamed to the smartphone, or which has been downloaded to the smartphone.
The noise-cancelling process is also known as active noise reduction or active noise control. The process includes sampling the ambient acoustical noise, producing a noise-containing electrical signal representative thereof, and transforming the noise-containing electrical signal into a noise-cancelling electrical signal by phase shifting the sinusoidally varying Fourier components of the noise-containing electrical signal substantially 180 degrees. The noise-cancelling electrical signal is then superimposed on a desired electrical signal representing music, speech, or other sounds of interest to the listener, such as may be transmitted to a device to which the noise-cancelling headphones are connected, such as a digital music player or a smartphone. The resultant electrical signal is amplified and used to drive the speakers in the headphones in place of the desired electrical signal. The resultant electrical signal is more advantageous for driving the headphones than the desired electrical signal because it is adapted to compensate for the acoustical noise whereas the desired electrical signal is not.
A microphone or equivalent sound transducer (hereinafter “microphone”) is required for sampling the acoustical noise, and electrical circuitry that typically includes an analog-to-digital converter (ADC), microprocessor, and a programmed or programmable memory for storing a program of instructions for the processor, transforms the noise-containing electrical signal into the noise-cancelling electrical signal.
In the prior art, the microphone and electrical circuitry is provided in the noise cancelling headphones, rendering them specialty items that are typically provided at higher cost. It is an objective of the present invention to provide for the same or similar noise-cancellation at a lower cost.